firmware: tegra: Fix locking bugs in BPMP

There are a bunch of error paths were we don't unlock the bpmp->threaded
lock.  Also if __tegra_bpmp_channel_write() fails then we returned
success instead of an error code.

Fixes: 983de5f9 ("firmware: tegra: Add BPMP support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>